This invention relates to fluorescent phosphors and more particularly to magnesium aluminum-gallate phosphors activated by manganese which can be excited by ultraviolet light and still more particularly to an improved method of making improved phosphors of the above type.
Such phosphors are known to emit narrow bands of green light in a manner similar to the emission of pure magnesium gallate activated by manganese as disclosed by Brown in U.S. Pat. No. 3,407,325. When excited by 253.7 nm ultraviolet radiation these phosphors are particularly applicable to xerographic reproduction techniques.
As xerographic reproduction has improved, new demands have been placed upon the phosphor-lamp combination. These demands have required not only improvements in phosphor-lamp brightness, phosphor-lamp maintenance (stability) and phosphor-lamp temperature dependence stability, but changes as well in the actual emission peak position in the spectrum for the luminescent material itself.
It is known that aluminum substitution for gallium in the magnesium gallate type phosphors will cause such an emission shift. As a matter of fact, concerning aluminum substitution in this type of phosphor, the entire spinel crystalline in the MgO-Al.sub.2 O.sub.3 -Ga.sub.2 O.sub.3 system has been studied and reported by Brown (Rf. Journal Electrochem. Soc. 114(3), 245-250 (1967). In general, aluminum substitution has been found to shift the peaks of the emission spectra of the phosphor materials activated by manganese to longer wavelengths (from about 510nm to 528nm). However, accompanying this spectral shift is a decrease in luminescence intensity (measured at total energy by integration of corrected relative spectral energy distribution curves) dropping to about 25% for a 50% aluminum substitution to virtually 0% for a 100% aluminum substitution. This significant drop in luminescence intensity with increased aluminum substitution essentially disqualified these aluminum-containing materials from commercial applications.
Concerning phosphor-lamp dependence stability, the effect of aluminum substitution for gallium in the synthesis of this type of phosphor is reported in British Pat. Nos. 1,159,324 and 1,242,983. Therein, temperature dependence stability has been shown to improve substantially for compositions defined by formulae of the type: EQU Mgx (Ga.sub.1.sub.-y Aly).sub.2 O.sub.3.sub.+x.sub.+z : M.sub.nz .sup.+.sup.+
wherein
0.97 .gtoreq. x .gtoreq.0.70 PA1 0.4 .gtoreq. y .gtoreq.0.025 and PA1 0.05 .gtoreq. z .gtoreq. 0.002 PA1 x+y+z = 1 PA1 0 .ltoreq. a .ltoreq. 1.0 PA1 0.002 .ltoreq. b .ltoreq. 0.06 PA1 0 .ltoreq. x .ltoreq. 0.96 PA1 0 .ltoreq. z .ltoreq. 1.00 PA1 0.75 .ltoreq. p .ltoreq. 1.10 PA1 Mo.sub.2 = geO.sub.2 and/or SiO.sub.2 PA1 0.70 .ltoreq. x .ltoreq. 1.05 PA1 0.025 .ltoreq. y .ltoreq. 0.4 PA1 0 .ltoreq. z .ltoreq. 0.3 PA1 0.001 .ltoreq. p .ltoreq. 0.05
And EQU p(Mg.sub.x Li.sub.0.5y Ga.sub.0.5y ZnO) . Ga.sub.2.sub.-a Al.sub.a O.sub.3. bMnO
wherein
and wherein 0.05 .ltoreq. a .ltoreq. 1.0 when 0.90 .ltoreq. z .ltoreq. 1.00.
The resulting temperature dependence stability noted for these materials is specified particularly for high-pressure mercury vapor lamp applications where the temperature experienced by the phosphors is often in excess of 300.degree.C. The above-cited British patents indicate a similar dramatic decrease in luminescence intensity with substitution of aluminum for gallium as originally shown by Brown; however, emphasis is placed on the fact that with superior temperature dependence stability, the aluminum substituted materials can exhibit superior luminescence intensity at higher temperatures than intensities observed for the more temperature sensitive pure magnesium gallate materials activated by manganese.
Concerning other modifications of the manganese activated magnesium gallate and aluminum-gallate phosphors, the effects on luminescence intensity and emission peak location in the spectrum for germanium and/or silicon substitution for gallium have been reported in British Pat. No. 1,248,373. In particular, luminescence intensity has been shown to be improved substantially for compositions defined by the formula: EQU xMgO . (1 .ltoreq. y .ltoreq. z)Ga.sub.2 O.sub.3 . yAl.sub.2 O.sub.3 . zMO.sub.2 : pMnO
wherein
Spectral shifts of the emission peaks of these materials were reported in the range from about 501nm to about 514nm for the above range of compositions.